This application is related to a concurrently filed patent application, Light Valve Projection System With Improved Vertical Resolution, Ser. No. 452,162, filed on behalf of T. True, W. Good and W. Bates.
The invention pertains to the field of light projection systems employing a deformable transparent light modulating medium which diffracts incident light to form projected images in accordance with modulating information. More particularly, the invention relates to such systems in which the light modulating medium is a fluid overlying a disk and information is written by an electron beam which scans out a raster on the fluid and forms orthogonally related diffraction gratings which control light rays transmitting the superimposed information necessary for color projection, systems of this type having been commonly termed light valve projection systems. The diffraction ratings are composed of one set of diffraction ratings parallel to the scanning lines containing, for example, green color information and two sets of diffraction gratings orthogonal to the scanning lines, in this example containing red and blue color information.
Light valve projection systems have been in commercial use for many years and are capable of providing good performance. Nevertheless, a number of problems have existed with respect to these systems of differing degrees of importance. A number of these problems such as those relating to the composition and processing of the fluid medium and overall light efficiency of the projected image have been substantially overcome. Another problem termed "burn-in" which produces a moire pattern of color, predominantly green, appearing within the projected picture has been solved by the invention of David A. Orser et al as described in U.S. Pat. No. 4,283,120. According to the Orser et al invention, a rotatable disk is mounted within the housing and has its lower portion immersed in a sump which holds the light modulating fluid so that the fluid covers over at least one surface of the disk during rotation. The raster is positioned on the disk offset from the coordinate axes so that movement of the fluid responsive to the disk rotation is along paths that intersect the diffraction gratings obliquely over the entire raster. Burn-in is eliminated because fluid motion never becomes tangential to either grating axis.
While the light valve projection system has been improved by the invention of David A. Orser et al, a parameter requiring improvement has been green vertical resolution. In the conventional approach, the green light is modulated by vertical diffractions caused by the grooves which the raster lines themselves write, and in order to produce a green "dark field" condition, the electron beam is wobbled so that adjacent raster lines just touch thereby producing a uniform charge density vertically. As green light is desired, the wobbling is decreased so that the charge distribution is changed causing the fluid film to deform so that some diffraction is produced. The decrease in wobbling is produced by negative amplitude modulation of the envelope of a wobbulating carrier. It is this negative amplitude modulation and consequent response of the control layer that causes lower than desired vertical resolution in the green channel. The low vertical resolution occurs because the light refracting sides of the grooves are displaced from the desired raster line location by as much as .+-.1 raster line spacing. For a green dark field, the wobble carrier spreads the charge for any one raster line over a vertical space equivalent to two raster line spaces. Subsequent modulation for a green light field decreases the carrier amplitude resulting in the detail information being displaced vertically from the raster line center. The result is the superposition of two images vertically displaced on either side of the desired location.